[unreadable] Assembly of a transcriptional activation complex occurs through cooperative recruitment of numerous proteins targeted to a specific DNA promoter sequence. Our long term goal is to understand the mechanism of allosterically regulated protein assembly as well as how these macromolecular machines regulate gene expression. As a model system, we are using the human progesterone receptor (PR). A mechanistic understanding of function is confounded by the presence of two functionally distinct isoforms, PR-A and PR- B. The two isoforms are identical except that the B-isoform contains an additional 164 amino acids at the N- terminus. Recent studies suggest that isoform specific differences may be due to differential recruitment of co-activating proteins such as steroid receptor coactivator-1 (SRC-1). We hypothesize that residues unique to the B-receptor allosterically regulate PR:isoform:SRC-1 affinities by modulating receptor structure. We will test this hypothesis by examining the energetics and structure of each isoform:SRC-1 complex on a DNA progesterone response element (PRE). We will use analytical ultracentrifugation, DNase footprinting, limited proteolysis, and CD spectroscopy to examine the interactions. [unreadable] [unreadable]